2.1.3 Assessment of Renal Function Flashcards
The use of the clearance of creatinine as a measure of GFR in patients:
is adequate under many conditions even though creatinine is filtered and secreted.
is not as accurate as the use of inulin clearance.
must be used with care when glomerular function is poor and tubular function is good.
All of the above.
All of the above.
Option d is correct. The clearance of creatinine is the most adequate way to determine GFR in renal patients. Creatinine is filtered by the glomerulus and secreted by the tubules. Its clearance exceeds the GFR by approximately 10%, therefore, for GFR determination it is not as accurate as the clearance of inulin, which is only filtered. Because creatinine is secreted, the tubules can compensate increasing its secretion if GFR is reduced. So, creatinine clearance may give a false estimation of GFR when glomerular function is poor.
What are the characteristics of postrenal failure?
glomerular pressure and functions are impaired
tubular function impaired in late stages
Decreased GFR
Decreased BUN/Cr and FENa > 3%
inability to concentrate urine
How does increasing plasma levels of glucose affect its clearance?
It will eventually increase it
What are the advantage of creatinine to measure GFR?
What is the equation for estimation of tubular secretion?
Estimation of renal secretion
The rate of tubular secretion (TS) of a solute by the tubules can be determined by the difference between the rate of its excretion, and the rate at which it is filtered:
GFR . PX + TS = (UX * V)
then,
TS = (UX * V) - (GFR * PX)
The following values were obtained from analysis of the blood and urine of a patient.
Hematocrit 43% The renal blood flow is:
To calculate RBF, renal plasma flow (RPF) should be estimated first. This can be obtained by measuring the clearance of Para-aminohippurate (PAH).
In this manner: RPF = UPAH . V / PPAH = 900 mg/ml . 2.36 ml/min / 3 mg/ml = 708 ml/min.
Now, to calculate the total blood flow, the hematocrit has to be taken into consideration.
RBF = RPF/(1-Hematocrit)
Remember that the value for the hematocrit has to be expressed as a fraction of 1.
Replacing in the equation above,
RBF = 708 ml/min/0.57 = 1,242.6 ml/min
Therefore, the correct answer is B.
The renal clearance of a substance that is filtered and secreted by the nephron:
increases as the concentration of the substance in plasma rises.
decreases as the concentration of the substance in plasma rises.
remains constant independent of its plasma concentration.
is not different from the clearance of a substance that is filtered and reabsorbed.
decreases as the concentration of the substance in plasma rises.
The clearance of a substance is affected by its concentration in plasma depending on how that substance is handled by the kidney. If that substance is filtered and secreted (for example creatinine), as its plasma concentration increases, more of it is filtered and eventually the transporters involved in its secretion in the tubules becomes saturated. Then its clearance will decrease (option b).
How can creatinine be used as a measure of renal disease progression?
Plasma creatinine concentration
Creatinine concentration in plasma is an indicator of renal function and because GFR is proportional to the reciprocal of serum creatinine, it provides a practical and useful way to estimate the progression and prognosis of renal disease. If the disease worsens, the reciprocal value will fall; if treatment is successful, 1/PCR will not decrease.
What are the characteristics of prerenal failure?
Glomerular function is impaired
Tubular function initially maintained
Decreased GFR
Increased BUN/Cr
Normal urine osmo
What is the equation for fractional excretion of water?
In the case of inulin how does clearance relate to GFR?
CIN = GFR
What are the general principles to measure renal function?
Consider the following example:
The following values were obtained from analysis of the blood and urine of a patient.
Based on these data, the diuresis in this patient is most probably the result of:
a decrease in the maximal transport for glucose.
a saturation of the transporters for glucose in the proximal tubule.
low concentration of circulating ADH.
The data do NOT indicate any potential causes for diuresis.
The data do NOT suggest a diuresis in this patient.
a saturation of the transporters for glucose in the proximal tubule.
This patient has a very high plasma glucose concentration. After filtering in the glomerulus, this most probably will overload the glucose transport systems of the proximal tubules. The high glucose concentration released in the urine will produce solute diuresis giving the patient the high urine flow rate.
Urea is a good estimate of?
Tubular function
Creatinine is good indicator for?
glomerular function
Which of the following explains why the clearance of creatinine is greater than the clearance of inulin?
Creatinine is filtered and reabsorbed by the renal tubules.
Creatinine is filtered and secreted by the renal tubules.
Creatinine is mainly reabsorbed by the renal tubules.
Creatinine is mainly secreted by the renal tubules.
Creatinine is filtered and secreted by the renal tubules.
Creatinine is cleared from plasma by both filtration and secretion. Therefore, its clearance is higher than that of inulin that is only cleared through filtration.
The renal clearance of glucose is:
higher than that of creatinine.
lower than that of inulin.
equal to the glomerular filtration rate.
independent of the glucose concentration in plasma.
Lower than inulin
The renal clearance of a substance depends on how the tubules handle that substance. For inulin that is only filtered, clearance equals the GFR. In contrast for a substance that is filtered and then completely reabsorbed, such as glucose, the clearance will be lower than the GFR or than the clearance of inulin (option b). Because the tubules have a maximal capacity for glucose transport, the system can be saturated. Therefore, until it reaches a maximum, glucose clearance is influenced by its concentration in plasma.
The following values were obtained from an individual.
What is the most accurate estimate of GFR?
GFR can be calculated using the clearance equation (GFR = Ux . V / Px). The most accurate estimate will be obtained by determining the clearance of a substance that is only filtered by the kidney, such as inulin. In this manner:
GFR = UIN . V / PIN = 130 mg/dl x 1.25 ml/min / 1.3 mg/ml = 125 ml/min.
What is the equation for estimation of tubular reabsorption
Estimation of renal reabsorption
Knowing GFR, the rate of tubular reabsorption from the filtrate or the rate of secretion into the tubular fluid of substances can be measured. The rate of tubular reabsorption (TR) of a solute by the tubules can be determined by the difference between the rate at which the substance is filtered and the rate at which it is excreted:
GFR . PX = (UX . V) + TR
then,
TR = (GFR . PX) – (UX . V)
Where PX and UX are the solute plasma and urine concentrations respectively.
The following values were obtained from an individual.
The renal plasma flow in this individual is:
RPF can be calculated using the clearance of Para-aminohippurate (PAH):
RPF = UPAH . V / PPAH = 2,500 mg/ml x 1.25 ml/min / 5 mg/ml = 625 ml/min.
This is a normal RPF value.
The following values were obtained from analysis of the blood and urine of a patient.
Regarding the fractional excretion of K+:
the patient is excreting an amount of K+ that is lower than that filtered.
the patient is excreting an amount of K+ that is within 10% of that filtered.
the patient is excreting an amount of K+ that is higher than that filtered.
the patient is excreting an amount of K+ that is higher than that filtered.
The fractional excretion of K+ can be obtained by using the following equation:
FEK= (U/P)K / (U/P)CR x 100 , replacing by the given values:
FEK=(120 mEq/L/1.7 mEq/L) / (48 mg/dL/0.9 mg/dL) x 100= 132%.
As an example
The following values were obtained from analysis of the blood and urine of a patient.
The GFR in this patient is:
To calculate GFR, the following equation is used:
GFR = UCR . V / PCR = 48 mg/dl x 2.36 ml/min / 0.9 mg/ml = 125.9 ml/min.
The obtained value is within the normal range.
What is the equation for fractional excretion of a solute?
Normal value of FENa is 1-3%
Consider the following example
How does GFR correspond to chronic renal disease?
The following values were obtained from an individual.
The fractional excretion of Na+ is:
The fraction of the filtered amount of Na+ that is excreted by the tubules or fractional excretion can be obtained by using the following equation:
FENa= (U/P)Na / (U/P)CR x 100 , replacing by the given values:
FENa=(115 mEq/L/143 mEq/L) / (100 mg/dL/0.9 mg/dL) x 100= 0.72%.
How will levels of creatinine change in response to differences in GFR?
As GFR decreases the plasma creatinine levels will increase
The following values were obtained from analysis of the blood and urine of a patient.
The fractional excretion of sodium (FENa) in this patient is:
The fraction of the filtered amount of a solute (Na+ in the present case) that is excreted by the tubules is known as fractional excretion. This can be obtained by using the following equation: FENa= (U/P)Na / (U/P)CR x 100.
Therefore, replacing by the given values:
FENa=(137 mEq/L/135 mEq/L) / (45 mg/dL/1.5 mg/dL) x 100= 3.38%.
What are the disadvantages to using creatinine to measure GFR?
How do you measure renal plasma flow?
How will clearance of a substance that is secreted change with increasing plasma concentration levels?
The clearance of a substance that is secreted falls when plasma concentration rises.
For a substance that is secreted, as its plasma concentration increases, the transport mechanism that secretes it becomes saturated, a smaller amount becomes secreted and the volume of plasma cleared of the substance falls.
